Two-stroke engine

The two-strokeinternal combustion engine differs from the more common four-strokeengine by completing the same four processes (intake, compression, combustion, exhaust) in only two strokes of the piston rather than four. This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform the intake and exhaust functions. This allows a power stroke for every revolution of the crank, instead of every second revolution as in a four-stroke engine. For this reason, two-stroke engines provide high specific power, so they are valued for use in portable, lightweight applications such as chainsaws as well as large-scale industrial applications like locomotives.

Invention of the two-stroke cycle is attributed to Dugald Clerk around 1880 whose engines had a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Joseph Day (and Frederick Cock for the piston-controlled inlet port).

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Unlike a petrol engine, which employs a spark plug to ignite the fuel/air charge in the cylinder, a Diesel engine relies solely on the heat of compression for ignition. Fuel is injected at high pressure into the superheated compressed air and instantly ignites. Therefore, scavenging is performed with air alone, combustion gases exiting through conventional poppet-type exhaust valves.

In order to allow the usage of a conventional oil-filled crankcase and pressure lubricated main and connecting rod bearings, modern two-stroke Diesels are scavenged by a mechanically driven blower (often a Roots positive displacement blower) or a hybrid turbo-supercharger, rather than by crankcase pumping. Generally speaking, the blower capacity is carefully matched to the engine displacement so that a slight positive pressure is present in each cylinder during the scavenging phase (that is, before the exhaust valves are closed). This feature assures full expulsion of exhaust gases from the previous power stroke, and also prevents exhaust gases from backfeeding into the blower and possibly causing damage due to contamination by particulates.

Early two-stroke Diesels using the crosshead layout (where the cylinder is not integral with the crankcase) employed under-piston pressure to provide scavenge air to the combustion chamber via a bypass port as used on a conventional gasoline-fueled two-stroke engine. Although the cross-head layout is still used on some large engines, greater power and efficiency, as well as lowered exhaust emissions, can be obtained with a mechanical blower or turbocharger.

It should be noted that the scavenging blower is not a supercharger, as its purpose is to supply airflow to the cylinders in proportion to their displacement and engine speed. A two-stroke Diesel supplied with air from a blower alone is considered to be naturally aspirated. In some cases, turbocharging may be added to increase mass air flow at full throttle—with a corresponding increase in power output—by directing the discharge of the turbocharger into the inlet of the blower, an arrangement that was found on some Detroit Diesel two-stroke engines.

A conventional, exhaust-driven turbocharger cannot be used by itself to produce scavenging airflow, as it is incapable of operating unless the engine is already running. Hence it would be impossible to start the engine. The common solution to this problem is to drive the turbocharger's impeller through a gear train and overrunning clutch. In this arrangement, the impeller turns at sufficient speed during engine cranking to produce the required airflow, thus acting as a mechanical blower. At lower engine speeds, the turbocharger will continue to act as a mechanical blower. However, at higher power settings the exhaust gas pressure and volume will increase to a point where the turbine side of the turbocharger will drive the impeller and the overrunning clutch will disengage, resulting in true turbocharging.